The present invention relates to a rotatable polygonal mirror used in an optical apparatus or the like having a laser scanning system, and to a manufacturing process thereof.
A polygonal mirror is usually a mirror that comprises a prism, for example, of a regular octahedral cross-section wherein the side faces thereof serve as reflective mirror faces; the mirror is used to deflect a laser beam to scan the surface that receives the beam.
Conventionally, aluminum alloy or optical glass has been used as a material of such a rotatable polygonal mirror. After being formed into a regular octahedral shape through a milling or grinding process, aluminum alloy is processed by a sintered carbide tool and optical glass is polished and reflective mirror faces are formed.
Polishing of glass requires a large number of processes to achieve the desired smoothness. Therefore, the cost is several times as high as that of metal milling. When metal is used, the material cost and the milling process cost are high. Metal is far from being comparable to molded mirror, with disadvantages such as scattering during optical scanning due to the curved surface made by cutting tools, cavities found in the aluminum material, lower operation efficiency caused by difficulties in processing, and the number of process steps required.
Since rotatable polygonal mirrors made of metal or glass are heavy, a complicated and expensive drive system and control system are required to realize high speed rotation. In addition, a tremendous amount of labor and time is necessary in the above-mentioned manufacturing process to obtain a highly precise rotatable polygonal mirror, which results in high manufacturing cost.
A synthetic resin has also been tried to form a rotatable polygonal mirror. Even if a synthetic resin is used, however, defects such as depressions or shrink marks occur when a mirror exceeds a certain thickness. If thinned portions are provided, ribs are necessary to increase mechanical strength. Then the mirror section will be deformed by centrifugal force at the time of high speed rotation. If injection molding of such a resin is adopted, it is difficult to maintain uniform molding pressure because of the complex shape of a rotatable polygonal mirror. As a result, internal stress causes warpage and depressions, which makes it almost impossible to obtain reflective mirror faces with sufficient mechanical strength and high flatness.